The present invention relates to auxiliary heating for motor vehicles with internal combustion engines.
To reduce fuel consumption, the efficiency of internal combustion engines is being continuously improved. As a consequence of this, the waste heat output of the motors, typically used to heat the passenger compartment, is decreasing. Particularly in the case of diesel-powered vehicles and hybrid vehicles, with a combination of an electro motor and an internal combustion engine, this leads to where the waste heat output is no longer sufficient for heating the passenger compartment, even during constant operation.
So-called auxiliary heaters are known which provide auxiliary heat during the cold start phase or also during partial load operation. Here, heat is produced by burning fuel with the aid of a flame burner. However, the disadvantage of this lies in increased fuel consumption as well as the formation of nitric oxides during the combustion process. Therefore, the use of such an auxiliary heater cancels out efforts towards improving engine efficiency. Moreover, additional problems arise while complying with emissions standards.
In order to comply with these limits, new vehicles are provided with emission control devices. They consist of a so-called converter housing which forms a part of the exhaust line of the vehicle. Inside the converter housing, emission control catalysts are provided which consist mostly of a monolithic block inside of which parallel flow channels for the exhaust gas are provided. The walls of the flow channels of these so-called honeycombs are provided with catalytically active coatings which convert harmful substances such as carbon monoxide, unburned hydrocarbons and nitric oxides contained in the exhaust gas of the combustion engine into carbon dioxide with a high degree of efficiency.
During the conversion of the harmful substances by the catalyst, combustion heat is released by the oxidation of carbon monoxide and the unburned hydrocarbons which also heat up the catalyst and the exhaust gas. When a large amount of harmful materials is present in the exhaust gas, the additional exothermal effect may lead to the destruction of the catalyst.
DE 39 20 159 A1 describes the heating of a motor vehicle by means of a catalytic converter with a heat exchanger. Here, the energy released during the catalytic conversion of a reaction mixture is utilized for heating purposes. Therefore, the catalytic converter is in the form of a heat exchanger or is located upstream from one. Prior to the start of the operation, the catalyst is heated by means of an electrical heater to the temperature necessary for conversion, whereby the output of harmful substances is minimized and simultaneously makes heat available very rapidly for heating purposes. By injecting fuel and by supplying combustion air before the catalytic converter, it may be used as a catalytic burner, whereby auxiliary heating is made possible as well.
The disadvantage of this solution is that it involves very cost-intensive technology. Electrical heating of the catalytic converter is required. If the catalytic converter is provided with an integrated heat exchanger, then its thermal mass is very large. This has corresponding negative effects on the heating-up rate and on the electrical energy required for this purpose. During cooling, as a result of the heat exchanger, there is the danger that the necessary reaction temperature is not attained and thereby a complete conversion of the harmful substances is not possible. However, if the heat exchanger is placed behind the catalytic converter instead, there exists the danger that in the case of fuel flooding, the catalyst can be thermally destroyed due to the high exothermal effects.
DE 43 40 463 A1 also describes the use of an exhaust device for heating a vehicle. Before the catalytic converter, a flame burner is provided which produces hot exhaust gases and assures the operational readiness of the catalyst as a result of the heating. To use the burner to heat the vehicle, a heat exchanger is provided which removes the energy from the exhaust gas flow between the burner and the catalytic converter. Also, downstream with respect to the catalytic converter, a heat exchanger may be provided.